Suppression of inclined defect formation and increase in critical thickness by silicon doping on non-c-plane (Al,Ga,In)N

ABSTRACT

A method for fabricating a III-nitride based semiconductor device, including (a) growing one or more buffer layers on or above a semi-polar or non-polar GaN substrate, wherein the buffer layers are semi-polar or non-polar III-nitride buffer layers; and (b) doping the buffer layers so that a number of crystal defects in III-nitride device layers formed on or above the doped buffer layers is not higher than a number of crystal defects in III-nitride device layers formed on or above one or more undoped buffer layers. The doping can reduce or prevent formation of misfit dislocation lines and additional threading dislocations. The thickness and/or composition of the buffer layers can be such that the buffer layers have a thickness near or greater than their critical thickness for relaxation. In addition, one or more (AlInGaN) or III-nitride device layers can be formed on or above the buffer layers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) ofand commonly-assigned U.S. Provisional Patent Application Ser. No.61/486,097, filed on May 13, 2011, by Matthew T. Hardy, Po Shan Hsu,Steven P. DenBaars, James S. Speck, and Shuji Nakamura, entitled“SUPPRESSION OF INCLINED DEFECT FORMATION AND INCREASE IN CRITICALTHICKNESS BY SILICON DOPING ON NON-C-PLANE (Al,Ga,In)N,”, whichapplication is incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned applications:

U.S. Utility application Ser. No. 12/861,532, filed on Aug. 23, 2010,now U.S. Pat. No. 8,481,991, issued on Jul. 9, 2013, by Hiroaki Ohta,Feng Wu, Anurag Tyagi, Arpan Chakraborty, James S. Speck, Steven P.DenBaars, Shuji Nakamura, and Erin C. Young, entitled “SEMIPOLARNITRIDE-BASED DEVICES ON PARTIALLY OR FULLY RELAXED ALLOYS WITH MISFITDISLOCATIONS AT THE HETEROINTERFACE,”, which application claims thebenefit under 35 U.S.C. Section 119(e) of U.S. Provisional ApplicationSer. No. 61/236,058, filed on Aug. 21, 2009, by Hiroaki Ohta, Feng Wu,Anurag Tyagi, Arpan Chakraborty, James S. Speck, Steven P. DenBaars, andShuji Nakamura, entitled “SEMIPOLAR NITRIDE-BASED DEVICES ON PARTIALLYOR FULLY RELAXED ALLOYS WITH MISFIT DISLOCATIONS AT THEHETEROINTERFACE,”;

U.S. Utility application Ser. No. 12/861,652, filed on Aug. 23, 2010, byHiroaki Ohta, Feng Wu, Anurag Tyagi, Arpan Chakraborty, James S. Speck,Steven P. DenBaars, Shuji Nakamura, and Erin C. Young, entitled“ANISOTROPIC STRAIN CONTROL IN SEMIPOLAR NITRIDE QUANTUM WELLS BYPARTIALLY OR FULLY RELAXED ALUMINUM INDIUM GALLIUM NITRIDE LAYERS WITHMISFIT DISLOCATIONS,”, which application claims the benefit under 35U.S.C. Section 119(e) of U.S. Provisional Application Ser. No.61/236,059, filed on Aug. 21, 2009 by Hiroaki Ohta, Feng Wu, AnuragTyagi, Arpan Chakraborty, James S. Speck, Steven P. DenBaars, and ShujiNakamura, entitled “ANISOTROPIC STRAIN CONTROL IN SEMIPOLAR NITRIDEQUANTUM WELLS BY PARTIALLY OR FULLY RELAXED ALUMINUM INDIUM GALLIUMNITRIDE LAYERS WITH MISFIT DISLOCATIONS,”;

U.S. Utility application Ser. No. 12/284,449 filed on Oct. 28, 2011, byMatthew T. Hardy, Steven P. DenBaars, James S. Speck, and ShujiNakamura, entitled “STRAIN COMPENSATED SHORT-PERIOD SUPERLATTICES ONSEMIPOLAR GAN FOR DEFECT REDUCTION AND STRESS ENGINEERING,”, whichapplication claims the benefit under 35 U.S.C. Section 119(e) ofco-pending and commonly-assigned U.S. Provisional Application Ser. No.61/408,280 filed on Oct. 29, 2010, by Matthew T. Hardy, Steven P.DenBaars, James S. Speck, and Shuji Nakamura, entitled “STRAINCOMPENSATED SHORT-PERIOD SUPERLATTICES ON SEMIPOLAR GAN FOR DEFECTREDUCTION AND STRESS ENGINEERING,”; and

U.S. Provisional Application Ser. No. 61/550,874, filed Oct. 24, 2011,by Po Shan Hsu, Matthew T. Hardy, Steven P. DenBaars, James S. Speck,and Shuji Nakamura, entitled “NONPOLAR/SEMIPOLAR (AL,IN,B,GA)N LASERSWITH STRESS RELAXATION AT THE P-CLADDING/P-WAVEGUIDING ANDN-CLADDING/N-WAVEGUIDING HETEROINTERFACES,”;

all of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related generally to the field of electronic andoptoelectronic devices, and more particularly, to suppression ofinclined defect formation and increase of critical thickness by Silicon(Si) doping on non-c-plane (Al,Ga,In)N.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [Ref. X]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

Despite recent progress, the performance of green light emitting diodes(LEDs) and laser diodes (LDs) is much lower than equivalent devicesemitting in blue or violet regimes. Active regions operating in thegreen regime require Indium (In) compositions in the quantum wells (QWs)of around 30%. Due to the large lattice mismatch between InN and GaN ofaround 10%, such structures must be grown at very high strain, e.g., 3%for In_(0.3)Ga_(0.7)N, thereby degrading crystal quality and leading tolarge piezoelectric induced electric fields in the quantum wells. Stressrelaxation also limits the composition and thickness of InGaNwaveguiding layers in LDs [Ref. 1].

For traditional planar c-plane and nonpolar strained heteroepitaxy,stress relaxation typically does not occur via slip due to the absencesof resolved shear stress on the c-plane, which is the most favorableslip system. However, c-plane slip has been observed on (20-21) and(11-22) semipolar orientations, which have significant resolved shearstress on the c-plane [Ref 2].

An available stress relaxation mechanism which preserves the crystalquality of overlying layers opens up the possibility of growing relaxedInGaN buffers. Aside from reducing active region strain during growth,an InGaN virtual substrate would open up device design space by allowingfor tensile strained or unstrained InGaN barriers to reduce bandoffsets, reduced piezoelectric polarization in the QWs, and increasedcritical thickness for InGaN waveguiding layers. Moreover, c-plane slipcan only relieve stress parallel to the c-plane, and at higher layerthicknesses and compositions, the stress in the orthogonal directionwill eventually lead to relaxation by another mechanism.

AlGaN films can relieve this strain via cracking, but this mechanism isnot available in InGaN due the compressive nature of the epi-strain. Atthicknesses sufficiently beyond the critical thickness, dark lines areobserved inclined with respect to the c-direction. For (20-21) InGaN,these lines are parallel to the intersection of the growth plane withinclined m-planes, and in (11-22) InGaN, lines have been observedparallel to the intersection of the growth plane and inclined m- anda-planes. These lines are likely misfit dislocations (MDs) formed byslip on the inclined m-planes and/or a-planes.

FIG. 1 is a cathodoluminescence (CL) image of 500 nanometers (nm) ofIn_(0.03)Ga_(0.97)N on a (20-21) GaN substrate, showing dark dots 100,and wherein c-plane and m-plane slip lines 102, 104, respectively, arevisible in the background, and m-plane slip lines 106 are visible in theforeground. In FIG. 1, inclined lines 104, 106 are at an angle less than90° with respect to the in-plane a-direction, and line 102 is parallelto the a-direction. As shown in FIG. 1, interaction between the misfitdislocation lines from c-plane and m-plane slip leads to threadingdislocation (TD) multiplication, as seen by the dark dots 100 decoratingthe inclined lines 104, 106. The dark dots 100 at the intersection ofthe c-plane and m-plane slip lines are newly formed TDs with a densitygreater than 1×10⁸ cm⁻², wherein substrate TD densities are on the orderof 0.5-1×10⁷ cm⁻².

Once formed, TDs are very difficult to remove and have deleteriousconsequences for subsequently grown devices, in contrast to the misfitdislocation lines formed by slip, which can easily be buried far fromthe active region and have negligible impact on device performance.Thus, avoiding TD formation is highly desired and critical to achievingimproved device performance using relaxed buffer layers.

Thus, there is a need in the art for improved methods for preventing theformation of additional TDs. The present invention satisfies this need.Specifically, the present invention shows the impact of Si doping ondefect morphology and prevention of additional TD formation.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesa method for fabricating a III-nitride based semiconductor device,comprising (a) growing one or more buffer layers on or above asemi-polar or non-polar GaN substrate, wherein the buffer layers aresemi-polar or non-polar III-nitride buffer layers; and (b) doping thebuffer layers so that a number of crystal defects in III-nitride devicelayers formed on or above the doped buffer layers is not higher than anumber of crystal defects in III-nitride device layers formed on orabove one or more undoped buffer layers.

The buffer layers can comprise InGaN, AlGaN, or GaN for example.

The buffer layers can be doped with a doping concentration of more than3×10¹⁸ cm⁻³. The buffer layers can be doped with Silicon (Si).

The method can further comprise forming one or more III-nitride devicelayers on or above and/or below the buffer layers. The III-nitridedevice layers can be coherently grown on or above the doped bufferlayers. One or more of the III-nitride device layers formed on or abovethe doped buffer layers, including an active region, can have athreading dislocation density of 10⁷ cm⁻² or less. An indium compositionof one or more of the III-nitride device layers formed on or above thedoped buffer layers, including the active region, can be at least 30%,or sufficient for the device to emit light having a peak intensity at awavelength in a green wavelength range or longer. The III-nitride devicelayers formed on or above the doped buffer layers can comprise one ormore waveguiding layers and/or one or more cladding layers.

The doping can reduce or prevent formation of misfit dislocation lines,including additional threading dislocations, e.g., parallel to inclinedm-plane or a-plane directions.

The doping can reduce or prevent formation of misfit dislocations withline directions inclined with respect to an in-plane m- or a-directionof the III-nitride device layers formed on or above the doped bufferlayers. The doping can prevent formation of threading dislocations(e.g., additional threading dislocations) during growth of the bufferlayers and/or the III-nitride device layers on or above the doped bufferlayers. The doping can reduce or prevent formation of misfit dislocationlines resulting from c-plane slip where the line direction is parallelto an in-plane m- or a-direction of the III-nitride device layers formedon or above the doped buffer layers.

The doping of the buffer layer (e.g., GaN buffer layer) with Si caninfluence or control extended defect morphology.

The doping and/or thickness and/or composition of the doped bufferlayers can be such that the doped buffer layers have a thickness near orgreater than their critical thickness for relaxation. The thickness orcomposition of the (In,Al)GaN layers can be such that the (In,Al)GaNlayers are near or greater than their critical thickness for relaxation.

The method can further comprise increasing or maximizing a criticalthickness of the buffer layers by the doping, or wherein the doping issuch that the critical thickness is thicker as compared to without thedoping.

The present invention also encompasses a device fabricated according tothis method.

The present invention further discloses a III-nitride basedsemiconductor device structure, comprising one or more buffer layers onor above a semi-polar or non-polar GaN substrate, wherein the bufferlayers are semi-polar or non-polar III-nitride buffer layers, and adoping concentration in the buffer layers is optimized such that anumber of crystal defects in III-nitride device layers formed on orabove the doped buffer layers is not higher than a number of crystaldefects in III-nitride device layers formed on or above one or moreundoped buffer layers.

The present invention can be used to fabricate optoelectronic (LEDs,LDs) or electronic devices such as a transistor or High ElectronMobility Transistor (HEMT), or solar cells.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a cathodoluminescence (CL) image of 500 nm ofIn_(0.03)Ga_(0.97)N on a (20-21) GaN substrate, wherein the scale is 2micrometers (μm) and the magnification is 12000.

FIG. 2( a) is a schematic of a sample epitaxial structure, and FIGS. 2(b)-(i) are a series of CL images showing defect morphology for thestructure, wherein the scale is 20 μm in FIGS. 2( b)-(e) and 10micrometers (μm) in FIGS. 2( f)-(i).

FIGS. 3( a)-(c) are example epitaxial structures, wherein the layerdoped with Si, to control the misfit dislocation morphology, isindicated with angled lines.

FIG. 4 is a flowchart illustrating a method of fabricating a devicestructure.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

As noted above, relaxation on semipolar GaN occurs initially by slip onthe c-plane [Ref 2]. As the layer composition and thickness move fartherbeyond the critical thickness, relaxation in the orthogonal directionbegins to occur by slip on the inclined m-planes, as seen in FIG. 1. Theinteraction of the two interaction mechanisms leads to the generation ofadditional TDs, which have deleterious effects on any devices grown ontop of this relaxed buffer layer (see, e.g., the cross-referencedapplications set forth above and Ref. 1)

The present invention uses controlled Si doping to suppress the onset ofm-plane slip, thereby preventing the formation of additional TDs. It hasalso been observed that Si doping reduces the degree of relaxation,which suggests that Si doping may be used for films grown near thecritical thickness to suppress the onset of relaxation.

The present invention can be used to help realize a relaxed buffer layeron semipolar (SP) GaN with low TD density. Such a layer can provide anInGaN/AlGaN pseudo-substrate, allowing reduction in the strain duringgrowth of subsequent InGaN/AlGaN layers. Reduced strain in quantum wells(QWs) may result in higher internal quantum efficiencies in (1) greenactive regions, due to reduced defect density during growth on InGaNrelaxed buffers, or (2) Ultraviolet (UV) active regions (due to reduceddefect density during growth on AlGaN relaxed buffers), for LEDs, LDs,and solar cells. Additionally, a decrease in strain will lead to highercritical thicknesses for waveguiding layers (i.e., InGaN waveguidinglayers on an InGaN buffer layer).

InGaN-based SP LDs can tolerate some relaxation in the n-InGaNwaveguide, as long as it occurs via c-plane slip. The appearance ofm-plane slip and subsequent TD formation causes a rapid deterioration ofactive region quality. In this case, elevated Si doping would preventm-plane slip and preserve the active region quality.

The addition of Si suppresses the onset of relaxation via slip, allowingn-InGaN waveguiding layers to be grown farther beyond their criticalthickness. This would improve the confinement factor without creatingadditional defects near the active region.

The present invention has experimentally demonstrated suppression ofm-plane and a-plane slip for InGaN growth on (20-21) and (11-22)semipolar planes. The present invention is also applicable to growth ofAlGaN doped with Silicon (AlGaN:Si).

The present invention demonstrates high quality relaxed buffer layersthat can be used to enhance device performance. Relaxed buffer layersemploying the present invention can be used to make higher performancedevices, including LEDs, LDs and solar cells. LDs with higherconfinement factors, due to coherently grown n-InGaN waveguiding layers,would have lower threshold current densities, and thus lower operatingcurrent and voltage.

Technical Description

FIG. 2( a) is a schematic of a sample epitaxial structure 200 accordingto an example of the present invention. The sample epitaxial structureincludes a (20-21) or (11-22) GaN substrate 202 and anIn_(0.06)Ga_(0.94)N layer 204 (having a thickness 206) grown on the(20-21) or (11-22) GaN substrate 202.

A series of samples were grown using horizontal flow Metal OrganicChemical Vapor Deposition (MOCVD) on (20-21) free standing GaNsubstrates 202. The composition was fixed at In_(0.06)Ga_(0.94)N to growlayer 204 with a thickness 206 of 225 nm, and a Si₂H₆ flow rate wasvaried among 0, 0.4, 1 and 2 standard cubic centimeters per minute(sccm), corresponding roughly to doping levels of 0, 3×10¹⁸ cm⁻³, 7×10¹⁸cm⁻³, and 2×10¹⁹ cm⁻³, in the In_(0.06)Ga_(0.94)N layer 204,respectively. A sample with an undoped (0 sccm) In_(0.06)Ga_(0.94)Nlayer 204 was grown to a thickness 206 of 200 nm, instead of a thickness206 of 225 nm as for the remainder of the series, and should, ifanything, show reduced signs of relaxation, as compared to the remainingsamples, based on thickness 206 alone.

FIGS. 2( b)-(i) are a series of CL images showing the defect morphologyon 200 nm thick 206 (20-21) InGaN 204 for: in FIG. 2( b), no Si doping;in FIG. 2( c), 0.4 sccm Si₂H₆ flow rate (˜3×10¹⁸ cm⁻³ doping level); inFIG. 2( d), 1 sccm flow rate (˜7×10¹⁸ cm⁻³ doping level); and in FIG. 2(e), 2 sccm flow rate (˜2×10¹⁹ cm⁻³ doping level); and on 200 nm thick(11-22) InGaN for: in FIG. 2( f), no Si doping; in FIG. 2( g), 1 sccmSi₂H₆ flow rate; in FIG. 2( h), 2 sccm Si₂H₆ flow rate; and in FIG. 2(i), 3 sccm Si₂H₆ flow rate.

As shown in FIGS. 2( b)-(i), there is a strong impact of Si doping levelon the defect morphology. With no doping, there is a very high densityof inclined m-plane slip lines 208, such that it is difficult to see thec-plane slip lines that form first on (20-21). With 0.4 sccm Si flow,the density of m-plane slip lines has been greatly reduced, such thatthere are only a few discrete lines visible, along with the c-plane slip210. For 1 sccm flow, there is only a single short m-plane segment 208visible, and at 2 sccm flow, there is only c-plane slip 210 remaining.

Si doped InGaN samples were characterized using on-axis reciprocal spacemapping (RSM). This technique uses the measured macroscopic tilt of theepilayer relative to the substrate to calculate plastic strain, degreeof relaxation (DoR), and composition. Equivalent InGaN composition(InGaN_(eq)) is used to compare the total relaxation of films withdifferent compositions, and is given by InGaN_(eq)=DoR x, wherein x isthe percentage of InN in the InGaN, and corresponds to the compositionof the “virtual substrate” created by the relaxed InGaN layer.

The results given in Table 1 below show that the addition of Si allowsInGaN_(eq) values equal to or greater than films without Si, whilepreventing inclined defect formation. There is some scatter in the datadue to run-to-run reproducibility issues, measurement error, andsubstrate dependant variations in the grown-in defect density. In Table1, x corresponds to the percentage of indium in the composition and Xindicates the presence of inclined defects. While the mechanism isunclear, it is possible that Si is influencing the Peierls forces in thesecondary m-plane/a-plane slip system and delaying relaxation in thesesystems. The Si doping may also influence the Peierls forces in theprimary c-plane slip system and reduce the overall DoR. Alternatively,n-doping may change the Fermi level in the relaxed buffer layer, whichmay, in turn, increase the Fermi level dependent formation energy ofmisfit dislocations. In either case, the addition of Si may serve tohinder slip on the c-plane, while providing a larger barrier to slip onthe inclined m-plane and a-plane. If a fully coherent structure isdesired, high Si doping may also prove to increase the criticalthickness for the onset of any relaxation.

TABLE 1 Relaxation results measured from epilayer tilt using RSMmeasurements Si₂H₆ flow Thickness Inclined (sccm) (nm) x (%) InGaN_(eq)(%) Defects^(a) 20-21 0 185 6.0 1.8 X 0.4 185 6.3 1.6 X 1 185 6.2 0.46 —2 185 6.3 0.96 — 2 185 8.9 4.3 X 3 185 8.0 2.2 — 3 185 9.1 4.3 slight x4 185 9 3.4 slight x 0 200 7.1 3.0 X 0.4 225 7.1 3.3 X 1 225 9.0 3.4 — 2225 6.3 1.4 — 11-22 0 200 6 X 1 200 5.9 1.8 — 2 200 5.5 1.4 — 3 200 5.73.9 — 4 200 5.5 2.0 — 3 300 5.7 1.5 — ^(a)‘X’ indicates the presence ofinclined defects, including those resulting from m-plane or a-planeslip.

The present invention's technology will allow much higher degree ofrelaxation to be achieved on SP GaN, without additional TD generation.In the simplest case, a single or graded buffer layer could be grownwith sufficient Si doping to completely suppress m-plane and a-planeslip. A full LD, LED, solar cell or other device structure could then begrown on the pseudo-substrate with reduced epitaxial strain in onedirection. Alternatively, the relaxed, Si doped layer could be used asthe n-waveguiding layer in a more conventional LD structure.

Device Structures

FIGS. 3( a)-(c) are examples of epitaxial structures according to one ormore examples of the present invention. FIG. 3( a) is a LD structure 300comprised of a GaN substrate 302, an n-type cladding (n-cladding) layer304, an n-type waveguiding (n-waveguiding) 306 layer, an active region308, a p-type waveguiding (p-waveguiding) layer 310, and a p-typecladding (p-cladding) layer 312, wherein the n-waveguiding layer 306 isa relaxed n-waveguiding layer 306.

FIG. 3( b) is an LD structure 314 comprised of a GaN substrate 316, abuffer layer 318, an n-cladding layer 320, an n-waveguiding layer 322,an active region 324, a p-waveguiding layer 326, and a p-cladding layer328, wherein the buffer layer 318 is a relaxed buffer layer 318.

FIG. 3( c) is a LED or solar cell structure 330 comprised of a GaNsubstrate 332, a buffer layer 334, an (optional) spacer layer 336, anactive region 338, and a p-cladding layer 340, wherein the buffer layer334 is a relaxed buffer layer 334. In each case, the layer doped with Sito control the misfit dislocation morphology is shown with angled lines342, i.e., the n-waveguiding layer 306 in FIG. 3( a), the relaxed bufferlayer 318 in FIG. 3( b), and the relaxed buffer layer 334 in FIG. 3( c).The layers 306, 318, and 334 have the doping with Silicon according tothe present invention, in order to control extended defect morphology inthe device structures 300, 314, and 330.

In the case of fully coherent structures, higher Si doping can allow therealization of higher critical thicknesses. This would allow LDs grownwith thicker or higher composition n-InGaN waveguiding layers 306,providing increased optical confinement and allowing the fabrication ofdevices with lower threshold current densities and lower operatingcurrents and voltages.

Possible Modifications and Variations

The devices 300, 314, 330 can be semipolar or nonpolar devices. Thesubstrates 302, 316, and 332 can be semipolar or nonpolar substrates.The device layers 304-312, 318-328, and 334-340 can be semipolar ornonpolar layers, or have a semipolar or nonpolar orientation (e.g.,layers 304-312, 318-328 can grown on or above each other and/or on orabove the semipolar or nonpolar planes of the substrate 302, 316, 332).

The active layers 308, 324, 338 can emit or absorb light (orelectromagnetic radiation) having a peak intensity at a wavelength in agreen wavelength range or longer (e.g., red or yellow light), or a peakintensity at a wavelength of 500 nm or longer. However, the presentinvention is not limited to devices 300, 314, 330 emitting or absorbingat particular wavelengths, and the devices 300, 314, 330 can emit orabsorb at other wavelengths. For example, the present invention isapplicable to ultraviolet, blue, yellow, and red light emitting devices300, 314 or solar cells 330.

The active layers 308, 324, 338 can have a thickness t₁ sufficientlythick, and have sufficiently high Indium composition, such that thelight emitting device emits, or the solar cell absorbs, the light havingthe desired wavelengths.

The light emitting or absorbing active layer(s) 308, 324, 338 caninclude Indium containing layers, such as InGaN layers (e.g., one ormore InGaN quantum wells with GaN barriers). The InGaN quantum wells canhave an Indium composition of at least 7%, at least 10%, at least 16%,or at least 30%, and a thickness or well width greater than 4nanometers, e.g., 5 nm, at least 5 nm, or at least 8 nm. However, thequantum well thickness can also be less than 4 nm, although it istypically above 2 nm thickness.

The waveguiding layers 306, 310, 322, 326 can comprise indium containinglayers such as one or more InGaN quantum wells with GaN barrier layers(e.g., Indium content of at least 30%). The cladding layers 304, 312,340 can comprise AlGaN and/or GaN layers, for example.

The III-nitride device layers 304-312, 318-328, and 334-340 can compriselayers that are coherently grown, non-coherently grown, or that arepartially or fully relaxed. For a layer X grown on a layer Y, for thecase of coherent growth, the in-plane lattice constant(s) of X areconstrained to be the same as the underlying layer Y. If X is fullyrelaxed, then the lattice constants of X assume their natural (i.e. inthe absence of any strain) value. If X is neither coherent nor fullyrelaxed with respect to Y, then it is considered to be partiallyrelaxed. In some cases, the substrate might have some residual strain.

The equilibrium critical thickness corresponds to the case when it isenergetically favorable to form one misfit dislocation at thelayer/substrate interface.

Experimental, or kinetic critical thickness, is always somewhat orsignificantly larger than the equilibrium critical thickness. However,regardless of whether the critical thickness is the equilibrium orkinetic critical thickness, the critical thickness corresponds to thethickness where a layer transforms from fully coherent to partiallyrelaxed.

Another example of critical thickness is the Matthews Blakeslee criticalthickness [3].

One or more of the device layers (e.g., 304-312, 318-328, and 334-340)can have a thickness that is high enough and/or composition, such that afilm, comprising all or one or more of the device layers, has athickness near or greater than the film's critical thickness forrelaxation.

One or more of the III-nitride device layers (e.g., 304-312, 318-328,and 334-340) can be thicker, and/or have a higher alloy composition(e.g., more Al, In, and/or B, or non-gallium element), as compared toIII-nitride device layers without the doped buffer layer 306, 318, 334.

For example, a total thickness t₁ of all the active layers e.g., 308,324, 338 (e.g., multi-quantum-well stack thickness) can be equal to, orgreater than, the critical thickness of a similar active layer grown ina structure without the doped buffer layer 306, 318, 334 of the presentinvention (e.g., equal to or greater than the critical thickness of asimilar active layer grown directly on a GaN layer/substrate/latticemismatched layer or on an undoped buffer layer).

For example, a total thickness t₂ of the n-type waveguiding layers (orp-type waveguiding layers) can be equal to, or greater than, thecritical thickness of similar n-type waveguiding layers (or similarp-type waveguiding layers) grown in a structure without the doped bufferlayer 306, 318, 334 of the present invention (e.g., equal to or greaterthan the critical thickness of similar waveguiding layers grown directlyon a GaN layer/substrate/lattice mismatched layer or on an undopedbuffer layer).

For example, a total thickness t₃ of the n-type cladding layers (orp-type cladding layers) can be equal to, or greater than, the criticalthickness for similar n-type cladding layers (or similar p-type claddinglayers) grown in a structure without the doped buffer layer of thepresent invention (e.g., equal to or greater than the critical thicknessof similar cladding layers grown directly on a GaNlayer/substrate/lattice mismatched layer or on an undoped buffer layer).

The achievable thickness of a layer depends on its strain relative tothe buffer layer on which it is grown. For the case of alloys grown onGaN, the strain is determined by the alloy composition. For example, inundoped In_(x)Ga_(1-x)N films with x=8%, m-plane slip can be observed asearly as 60 nm thickness. With ˜3×10¹⁹ cm⁻³ Si doping, m-plane slip isnot observed even at 185 nm thickness. In this case, in terms of theMatthews-Blakeslee critical thickness h_(c), m-plane slip occurred at˜2h_(c) without Si doping and at greater than 6h_(c) with high Sidoping. The onset of c-plane slip in this composition regime is oftenaround 1.5h_(c). Calculations for the theoretical Matthews-Blakesleeh_(c) can be found in [Ref. 2].

Process Steps

FIG. 4 illustrates a method for fabricating an (AlInGaN) or III-nitridebased semiconductor device. The method may comprise the following steps.

Block 400 represents growing one or more buffer layers, e.g., on orabove a semi-polar or non-polar III-nitride or GaN substrate, whereinthe buffer layers are semi-polar or non-polar III-nitride or GaN bufferlayers.

Block 402 represents doping the buffer layers. The doping (e.g., dopinglevel and dopant) can be so that a number of crystal defects (e.g.,extended defects, threading dislocations, stacking faults) inIII-nitride device layers formed on or above the doped buffer layers isnot higher than a number of crystal defects in III-nitride device layersformed on or above one or more undoped buffer layers. For example, thestep can comprise doping a semi-polar or non-polar buffer layer (e.g.,GaN buffer layer) to influence or control extended defect morphology.

The buffer layers can be doped with a doping concentration of more than3×10¹⁸ cm⁻³. The buffer layers can be doped with Silicon (Si).

The doping can reduce or prevent formation of misfit dislocation linesparallel to the inclined m-plane or a-plane directions.

The doping can reduce or prevent formation of misfit dislocations withline directions inclined with respect to an in-plane m- or a-directionof the III-nitride device layers formed on or above the doped bufferlayers. The MDs the present invention can prevent here are formed fromm-plane or a-plane slip. The MD line direction is the intersection ofthe slip plane with the growth plane. In this case the line direction isinclined with respect to the in-plane a- or m-direction (depending onthe semipolar plane).

The doping can prevent formation of additional threading dislocationsduring growth of the buffer layers and/or the III-nitride device layerson or above the doped buffer layers.

The doping can reduce or prevent formation of misfit dislocation linesparallel to the c-plane direction.

The doping can reduce or prevent formation of misfit dislocation linesresulting from c-plane slip where the line direction is parallel to anin-plane m- or a-direction of the III-nitride device layers formed on orabove the doped buffer layers. The MDs result from c-plane slip and format the intersection of the c-plane and semipolar plane, which is thea-direction for (20-21) and the m-direction for (11-22).

The buffer layers can comprise GaN, InGaN, AlGaN, or AlInGaN, forexample.

The doping and/or thickness and/or composition of the buffer layers canbe such that the doped buffer layers have the thickness near or greaterthan their critical thickness for relaxation.

The doping can increase or maximize a critical thickness of the bufferlayer. For example, the doping can be such that the critical thicknessis thicker as compared to without the doping.

If the suppression of m-plane slip (and/or c-plane slip) is due to achange in the Fermi level, then any intentional or unintentional n-typedopant will show the same effect. Silicon is the most common intentionaldopant, but oxygen is a common unintentional dopant (especially in Alcontaining layers). Other n-type dopants include Se and Ca. In addition,growth conditions that enhance n-type intrinsic defects, such asnitrogen vacancies may contribute to this effect.

If m-plane slip suppression is due to impurity hardening effects, thenany n-type, p-type or non-electrically active impurity can be used,including Mg, Zn, Be, C and many other elements.

Block 404 represents forming (e.g., growing or depositing) one or more(In,Al)GaN or III-nitride layers (e.g., device layers) on or aboveand/or below the doped buffer layers. The step can comprise forming oneor more (AlInGaN) device layers on or above and/or below the (In,Al)GaNlayers.

A thickness or composition of the (In,Al)GaN layers can be such that the(In,Al)GaN layers are near or greater than their critical thickness forrelaxation.

One or more of the III-nitride device layers formed on or above and/orbelow the doped buffer layers, including an active region, can have athreading dislocation density of 10⁷ cm⁻² or less. The III-nitridedevice layers can be coherently grown on or above the doped bufferlayers. An indium composition of one or more of the III-nitride devicelayers formed on or above and/or below the doped buffer layers,including the active region, can be at least 30% or sufficient for thedevice to emit light peak intensity at a wavelength in a greenwavelength range or longer.

The III-nitride device layers formed on or above and/or below the dopedbuffer layers can comprise one or more waveguiding layers and/or one ormore cladding layers.

Block 406 represents the end result of the method, a device structure.The device structure can comprise, as illustrated in FIG. 2( a) and FIG.3( a)-(c), one or more layers 204, 318, 334 on or above a semi-polar ornon-polar GaN substrate 202, 316, 332 wherein the layers 318, 334 can besemi-polar or non-polar III-nitride buffer layers, and a dopingconcentration in the buffer layers 318, 334 is optimized such that anumber of crystal defects in III-nitride device layers 320-328, 338-340formed on or above the doped buffer layers 318, 334 is not higher than anumber of crystal defects in III-nitride device layers formed on orabove one or more undoped buffer layers.

The device can comprise an (AlInGaN) based semiconductor devicecomprising one or more (In,Al)GaN layers overlying a semi-polar ornon-polar GaN substrate, wherein the (In,Al)GaN layers employ doping toinfluence or control crystal defects or extended defect morphology. Oneor more (AlInGaN) device layers can be on or above and/or below the(In,Al)GaN layers. The thickness and/or composition of the (In,Al)GaNlayers overlying the semi-polar or non-polar GaN substrate can be (e.g.,high enough) such that the device structure or the (In,Al)GaN layers arenear or greater than their critical thickness for relaxation.

The device structure can be an LED, LD, or solar cell device structureas shown in FIGS. 3( a)-3(c). For example, the device layers can be thelayers of an LED (including an n-type layer, p-type layer, and activeregion), solar cell, or LD. The device layers of an LD can includecladding layers, waveguiding layers and an active region. The devicelayers can also be the layers of an electronic device such as atransistor.

Nomenclature

The terms “(AlInGaN)” “(In,Al)GaN”, or “GaN” as used herein (as well asthe terms “III-nitride,” “Group-III nitride”, or “nitride,” usedgenerally) refer to any alloy composition of the (Ga,Al,In,B)Nsemiconductors having the formula Ga_(w)Al_(x)In_(y)B_(z)N where 0≦w≦1,0≦x≦1, 0≦y≦1, 0≦z≦1, and w+x+y+z=1. These terms are intended to bebroadly construed to include respective nitrides of the single species,Ga, Al, In and B, as well as binary, ternary and quaternary compositionsof such Group III metal species. Accordingly, it will be appreciatedthat the discussion of the invention hereinafter in reference to GaN andInGaN materials is applicable to the formation of various other(Ga,Al,In,B)N material species. Further, (Ga,Al,In,B)N materials withinthe scope of the invention may further include minor quantities ofdopants and/or other impurity or inclusional materials.

Many (Ga,Al,In,B)N devices are grown along the polar c-plane of thecrystal, although this results in an undesirable quantum-confined Starkeffect (QCSE), due to the existence of strong piezoelectric andspontaneous polarizations. One approach to decreasing polarizationeffects in (Ga,Al,In,B)N devices is to grow the devices on nonpolar orsemipolar planes of the crystal.

The term “nonpolar plane” includes the {11-20} planes, knowncollectively as a-planes, and the {10-10} planes, known collectively asm-planes. Such planes contain equal numbers of Group-III (e.g., gallium)and nitrogen atoms per plane and are charge-neutral. Subsequent nonpolarlayers are equivalent to one another, so the bulk crystal will not bepolarized along the growth direction.

The term “semipolar plane” can be used to refer to any plane that cannotbe classified as c-plane, a-plane, or m-plane. In crystallographicterms, a semipolar plane would be any plane that has at least twononzero h, i, or k Miller indices and a nonzero 1 Miller index.Subsequent semipolar layers are equivalent to one another, so thecrystal will have reduced polarization along the growth direction.

REFERENCES

The following references are incorporated by reference herein:

-   1. A. Tyagi, F. Wu, E. C. Young, A. Chakraborty, H. Ohta, R.    Bhat, K. Fujito, S. P. DenBaars, S. Nakamura and J. S. Speck, Appl.    Phys. Lett. 95 251905 (2009).-   2. E. C. Young, C. S. Gallinat, A. E. Romanov, A. Tyagi, F. Wu    and J. S. Speck, Appl. Phys. Express 3 111002 (2010).-   3. J. Matthews and A. Blakeslee, J. Cryst. Growth 32 265 (1976).

CONCLUSION

This concludes the description of the preferred embodiments of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A method for fabricating a Group III-nitridebased semiconductor device, comprising: (a) growing one or more bufferlayers on or above a semi-polar or non-polar GaN substrate, wherein thebuffer layers are semi-polar or non-polar Group III-nitride bufferlayers; and (b) doping the buffer layers so that a number of crystaldefects in Group III-nitride device layers formed on or above the dopedbuffer layers is not higher than a number of crystal defects in GroupIII-nitride device layers formed on or above one or more undoped bufferlayers.
 2. The method of claim 1, wherein the buffer layers are dopedwith a doping concentration of more than 3×10¹⁸ cm⁻³.
 3. The method ofclaim 1, wherein the buffer layers are doped with Silicon (Si).
 4. Themethod of claim 1, further comprising forming one or more GroupIII-nitride device layers above the buffer layers.
 5. The method ofclaim 1, wherein the Group III-nitride device layers are coherentlygrown on or above the doped buffer layers.
 6. The method of claim 1,wherein one or more of the Group III-nitride device layers formed on orabove the doped buffer layers, including an active region, have athreading dislocation density of 10⁷ cm⁻² or less.
 7. The method ofclaim 6, wherein an indium composition of one or more of the GroupIII-nitride device layers formed on or above the doped buffer layers,including the active region, is at least 30%, or sufficient for thedevice to emit light having a peak intensity at a wavelength in a greenwavelength range or longer.
 8. The method of claim 7, wherein the GroupIII-nitride device layers formed on or above the doped buffer layerscomprise one or more waveguiding layers or one or more cladding layers,or the one or more waveguiding layers and the one or more claddinglayers.
 9. The method of claim 1, wherein the doping reduces or preventsformation of misfit dislocations with line directions inclined withrespect to an in-plane m- or a-direction of the Group III-nitride devicelayers formed on or above the doped buffer layers.
 10. The method ofclaim 9, where the doping prevents formation of additional threadingdislocations during growth of the buffer layers and the GroupIII-nitride device layers on or above the doped buffer layers.
 11. Themethod of claim 1, wherein the doping reduces or prevents formation ofmisfit dislocation lines resulting from c-plane slip where the linedirection is parallel to an in-plane m- or a-direction of the GroupIII-nitride device layers formed on or above the doped buffer layers.12. The method of claim 1, wherein the doping and a thickness, acomposition, or the thickness and the composition, of the doped bufferlayers are such that the doped buffer layers have a thickness near orgreater than their critical thickness for relaxation.
 13. The method ofclaim 1, further comprising increasing or maximizing a criticalthickness of the buffer layers by the doping, or wherein the doping issuch that the critical thickness is thicker as compared to without thedoping.
 14. A device fabricated according to the method claim
 1. 15. Themethod of claim 1, wherein the buffer layers comprise InGaN.
 16. Themethod of claim 1, wherein the buffer layers comprise AlGaN.
 17. A GroupIII-nitride based semiconductor device structure, comprising: one ormore buffer layers on or above a semi-polar or non-polar GaN substrate,wherein: the buffer layers are semi-polar or non-polar Group III-nitridebuffer layers, and a doping concentration in the buffer layers isoptimized such that a number of crystal defects in Group III-nitridedevice layers formed on or above the doped buffer layers is not higherthan a number of crystal defects in Group III-nitride device layersformed on or above one or more undoped buffer layers.
 18. The devicestructure of claim 17, wherein the doping concentration is more than3×10¹⁸ cm⁻³.
 19. The device structure of claim 17, wherein the bufferlayers are doped with Silicon (Si).
 20. The device structure of claim17, further comprising one or more Group III-nitride device layersformed above the buffer layers.
 21. The device structure of claim 17,wherein the Group III-nitride device layers are coherently grown on orabove the doped buffer layers.
 22. The device structure of claim 17,wherein one or more of the Group III-nitride device layers formed on orabove the doped buffer layers, including an active region, have athreading dislocation density of 10⁷ cm⁻² or less.
 23. The devicestructure of claim 22, wherein an indium composition of one or more ofthe Group III-nitride device layers formed on or above the doped bufferlayers, including the active region, is at least 30% or sufficient forthe device to emit light having a peak intensity at a wavelength in agreen wavelength range or longer.
 24. The device structure of claim 23,wherein the Group III-nitride device layers formed on or above the dopedbuffer layers comprise one or more waveguiding layers or one or morecladding layers, or the one or more waveguiding layers and the one ormore cladding layers.
 25. The device structure of claim 17, wherein thedoping concentration reduces or prevents formation of misfitdislocations with line directions inclined with respect to an in-planem- or a-direction of the Group III-nitride device layers formed on orabove the doped buffer layers.
 26. The device structure of claim 25,where the doping concentration prevents formation of additionalthreading dislocations in the Group III-nitride device layers on orabove the doped buffer layers.
 27. The device structure of claim 17,wherein the doping concentration reduces or prevents formation of misfitdislocation lines resulting from c-plane slip where the line directionis parallel to the in-plane m- or a-direction.
 28. The device structureof claim 17, wherein the doping concentration and a thickness orcomposition of the doped buffer layers is such that the doped bufferlayers are near or greater than their critical thickness for relaxation.29. The device structure of claim 17, wherein the buffer layers compriseInGaN.
 30. The device structure of claim 17, wherein the buffer layerscomprise AlGaN.
 31. The device structure of claim 17, wherein the GroupIII-nitride device layers formed on or above the doped buffer layers arethe layers of a laser diode (LD), light emitting diode (LED), solarcell, or transistor.
 32. The device structure of claim 17, wherein theGroup III-nitride device layers formed on or above the doped bufferlayers are layers of a laser diode, including cladding layers,waveguiding layers, and an active region.